Method and device for determining a parameter for an instantaneously maximal frictional force coefficient

ABSTRACT

The present invention relates to a method and a device for determining a characteristic quantity for an instantaneously maximum coefficient of friction between the tires of a vehicle and a roadway, which is calculated from the ratio between the frictional force and the wheel contact force, especially for defining a following time of the vehicle in relation to another vehicle which is directly in front thereof in the direction of travel, for collision avoidance purposes. In this arrangement, the characteristic quantity of the coefficient of friction is assigned to one of at least two classes and is then presented to a driver of the vehicle and/or sent to the collision avoidance system of the vehicle, by logically linking data output values of sensor means provided in the vehicle, on the one hand. On the other hand, there is provision of an evaluating device which is connected to an outside temperature sensor connected preferably to the engine control of the vehicle, to an air humidity sensor which is preferably connected to an air conditioning system of the vehicle, and/or to a wiper system, especially said&#39;s actuating button, stepped switch, and/or rain sensor, in said&#39;s inlet range, and to an indicator device, such as a display of a navigation system of the vehicle, and/or a collision avoidance system of the vehicle in its outlet range.

TECHNICAL FIELD

The present invention generally relates to vehicle stability control andmore particularly relates to a method and a device for determining acharacteristic quantity for an instantaneously maximum coefficient offriction between the tires of a vehicle and a roadway.

BACKGROUND OF THE INVENTION

It is known in the art to invariably adjust the safety distance betweentwo vehicles driving one behind the other in driving control(ICC—Intelligent Cruise Control) operations, i.e., generally to adistance in meters which corresponds to half the indication given on thespeedometer in kilometers per hour so that, for example, at 100 km/h thesafety distance amounts to 50 m. A safety distance of 50 m is identicalto a constant following time t_(s)=1.8 sec. ,irrespective of speed,which is referred to as standard following time hereinbelow.

When the time value drops below the value of the standard following timeof more than 50%, that means, t_(s)≦0.9 sec., this is presently fined.When the standard following time is exceeded to a considerable extent sothat t_(s)≧3 sec., this causes problems with filtering in. For thisreason, it has already been disclosed to render the following timemanually adjustable by the driver of a vehicle so that 0.9 sec.≦t_(s)<3sec. However, this suggestion must be looked at critically because inthe event of the driver and/or the weather changing, an incorrectlyadjusted value may cause too short stopping distances in criticalsituations, with the imminent risk of running from behind into thevehicle in front.

Further, it is known in the art that the instantaneously maximumcoefficient of friction which is given in approximation by thecoefficient of the longitudinal force between the tires of a vehicle anda roadway is appropriate to calculate a safety distance and a followingtime, respectively.

FIG. 1 shows a characteristic curve of the coefficient of friction/slip(μ-S-variation) for a conventional tire on a conventional roadway duringstraight travel at 80 km/h and an outside temperature of 20° C., on theone hand, with a height of water of 0.3 mm on the roadway, see curve I,and, on the other hand, with a height of water of 3 mm on the roadway,see curve II. The slip, i.e., the ratio of the difference between asynchronous speed (wheel translational speed) and an asynchronous speed(wheel circumferential speed) with respect to the synchronous speed isindicated in percent both for the drive side and the brake side. As canbe taken from a comparison of curves I and H, the coefficient offriction declines by 50% in the event of heavy rain, which should belinked to doubling the following time for safety reasons. Besides, itcan be seen in FIG. 1 that depending on the weather conditions, amaximum coefficient of friction, such as μ_(I) or μ_(II), respectively,prevails and must be demanded in emergency situations, and that with100% slip the coefficient of friction approaches a standard value μ_(G)in the event of slightly wet conditions which is a significant parameterin road planning and results from a rating with a single vehicle withPLARC tires traveling in a longitudinal direction on a wet but cleanroadway. Finally, it should still be pointed out that for safety reasonsa coefficient of friction μ=0.3 is demanded in conventional travelcontrol systems.

Up to date, there are two different approaches in determining theinstantaneously maximum coefficient of friction. Thus, optical scanningof a roadway surface beneath a vehicle and in front of a vehicle isperformed for the subsequent analysis of the corresponding refractionand reflection behavior, on the one hand. On the other hand, it has beendisclosed to install sensors for measuring the shearing forcerespectively the shearing deformation in the tread bar of a tire. Thesetypes of sensing the instantaneously maximum coefficient of friction arecostly in terms of manufacture and installation.

EP 0 412 791 A2 describes a method of observing and determiningconditions between a vehicle and the roadway surface wherein signals ofseveral sensors for the roadway surface, the vehicle, and outsideconditions are analyzed and compared, and a value of the coefficient offriction is determined therefrom.

An object of the present invention is to provide a characteristicquantity for the instantaneously maximum coefficient of friction in asimple and inexpensive manner.

This object is achieved by a method of determining a characteristicquantity for an instantaneously maximum coefficient of friction betweenthe tires of a vehicle and a roadway which is calculated from the ratiobetween the frictional force and the wheel contact force to define afollowing time of the vehicle, which is equivalent to a safety distance,in relation to another vehicle which is directly in front thereof in thedirection of travel, for collision avoidance purposes, wherein bylogically linking data output values of sensor means provided in thevehicle, the characteristic quantity of the coefficient of friction isassigned to one of at least two classes and is then presented to adriver of the vehicle and/or sent to the collision avoidance system ofthe vehicle.

In a preferred aspect of the present invention, the characteristicquantity of the coefficient of friction is assigned to a first class inthe event of wet road conditions and otherwise to a second class,preferably, in response to the speed of the vehicle.

In this arrangement, the speed-responsive characteristic quantity of thecoefficient of friction of each of the two classes is determined by wayof the frequency distribution function of the coefficients of frictionover a large quantity of roadways, such as over all German roads,measured at a 100% slip value, especially for a single vehicle withstandard tires traveling in a longitudinal direction on a wet but cleanroadway, preferably over the 95% sum frequency curve of said frequencydistribution function.

A preferred aspect of the present invention includes that thecharacteristic quantity of the coefficient of friction, by way of anindistinct logic, is assigned to one of three classes and, thus, to oneof three characteristic quantities which are preferably irrespective ofspeed.

It may be provided that a characteristic quantity of the frictionalforce is assigned to one of the three classes and, thus, to one of threecharacteristic quantities in dependence on parameters of the tires ofthe vehicle, the roadway, and/or the contact medium between the tire andthe roadway, wherein at least one estimation of the parameters of thecontact medium is performed and, preferably, the wheel contact force iscalculated in approximation for the classification.

Besides, it is proposed according to the present invention that theoutside temperature, the relative air humidity, and/or the wiperadjustment, such as on/off and/or frequency, is/are sensed and usedlogically as the parameters of the contact medium, especially with theassumption of an average roadway and average tires, to determine thecharacteristic quantity of the frictional force.

It may also be provided according to the present invention that the dataoutput value of a rain sensor is taken into consideration as a parameterof the contact medium when determining the characteristic quantity ofthe frictional force.

Further, the present invention discloses that low frictional forces orcoefficients of friction, respectively, are represented by a firstcharacteristic quantity, medium frictional forces or coefficients offriction are represented by a second characteristic quantity, and highfrictional forces or coefficients of friction are represented by a thirdcharacteristic quantity, and preferably a first following time of e.g.2.5 seconds is assigned to the first characteristic quantity, a secondfollowing time of e.g. 1.8 seconds is assigned to the secondcharacteristic quantity, and a third following time of e.g. 1.3 secondsis assigned to the third characteristic quantity, the said assignmentsbeing made irrespective of speed.

Also, it is disclosed in the present invention that the type of tire,tread design, tire print, rubber compound, and/or the condition of wearis/are taken into consideration as parameters of the tires of thevehicle in the classification for the instantaneously maximumcoefficient of friction.

It may be provided that the type of tires can be input by the driver orread in automatically, preferably, by way of a coding.

One embodiment of the present invention is characterized in that thecondition of wear of each tire of the vehicle is detected by way of awear model, preferably, in consideration of a speed histogram, asrelated to the wheel rotational speeds, in conjunction with a transverseacceleration histogram, as detected by transverse acceleration sensors,and in conjunction with a coefficient-of-friction histogram.

It is also possible according to the present invention to determine thecondition of wear in consideration of a histogram of the wheelpressures.

Also, according to the present invention, the material of construction,the structure of the surface and/or the surface temperature is/are takeninto consideration as parameters of the road surface in theclassification for the instantaneously maximum coefficient of friction.

Further, the height of a water film, ice and/or solid snow on a roadwayis/are taken into consideration as parameters of the contact mediumaccording to the present invention.

Still further, the parameters of the roadway and/or of the contactmedium are input into an evaluating device by way of a transpondersystem at the border of a roadway according to the present invention.

To achieve the object of the present invention, an improvement of thegeneric device is disclosed that is characterized by an evaluatingdevice which is connected to an outside temperature sensor preferablylinked to the engine control of the vehicle, to an air humidity sensorwhich is preferably connected to an air conditioning system of thevehicle, and/or a wiper system, especially said's actuating button,stepped switch and/or rain sensor, in said's inlet range, and to anindicator device, such as a display of a navigation system of thevehicle, and/or a collision avoidance system of the vehicle in itsoutlet range.

It may be provided that the evaluating device is connected to a roadsurface temperature sensor, such as an infrared camera.

Besides, the present invention discloses that the evaluating devicereceives parameters of a roadway and/or a contact medium between theroadway and the tires of the vehicle by means of telemetric data inputfrom transponders at the border of the roadway.

One embodiment of the present invention is characterized in that speeddata to determine a speed histogram, transverse acceleration data todetermine a transverse acceleration histogram, and/or coefficients offriction to determine a wheel pressure histogram can be stored in theevaluating device.

In addition, the present invention discloses that the evaluating deviceis connected to an electric-hydraulic brake system to obtain wheelpressures, and the wheel pressures can be stored in the evaluatingdevice for determining a wheel pressure histogram.

The present invention also proposes a connection between the evaluatingdevice and a control panel for the manual input of parameters.

Finally, it is still disclosed in the present invention that theevaluating device is connected to a device for reading in a type oftires, such as a scanning sensor, preferably comprised on a springstrut, for detecting a coding pulse, preferably by means of a magneticcoding.

Thus, the present invention is based on the surprising finding thatsensor data which are already provided in a vehicle are logicallycombined with each other so that an estimation for the instantaneouslymaximum coefficient of friction is obtained, from which e.g. a followingtime is calculated and relayed to a driver or is sent to a collisionavoidance of a cruise control system for effectively enhancing thesafety of driving.

An improvement of the present invention, which is convincing especiallyin its simplicity, founds on the principle ‘wet road - take off footfrom the accelerator pedal’, that means, there is a classification inone of two speed-responsive coefficient-of-friction classes, the onlydistinction made being between wet conditions and none-wet conditions,and reference being made to the frequency distribution function of μ_(G)over all German roads, especially the 95% sum frequency.

The preferred embodiment according to the present invention is based onan indistinct logic, the so-called fuzzy logic, according to which acharacteristic quantity is assigned to the instantaneously maximumcoefficient of friction, and namely in such a manner that there is aclassification into one of three classes, to which a characteristicquantity is respectively assigned. The respective characteristicquantity for the instantaneously maximum coefficient of friction maythen be presented on a display to a driver for a recommendation ofadjustment or can be sent to a travel control system for the automaticadaption of the following distance.

It is preferably proposed to make an at least rough estimation of theinfluence of the contact medium between the vehicle and the roadway forthe determination of the characteristic quantity of the instantaneouslymaximum coefficient of friction according to the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows characteristic curves of the instantaneously maximumcoefficient of friction in relation to the slip for a conventional tireon a conventional road during straight travel at 80 km/h and an outsidetemperature of 20° C., with a height of water of 0.3 mm and 3 mm.

FIGS. 2a, 2 b respectively show the course of the following time as afunction of the driving speed, with wet conditions in FIG. 2a, and withnone-wet conditions in FIG. 2b, according to an embodiment of thepresent invention.

FIG. 3 shows a condition linking diagram to illustrate a possibleassignment of measured values to an instantaneously maximum coefficientof friction or, respectively, a following time based on a fuzzy logicaccording to another embodiment of the present invention.

FIG. 4 is a partial cross-sectional view for a demonstration of sensinga type of tire.

FIG. 5 is a block diagram of an evaluating device of the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

According to a first embodiment of the present invention, a distinctionis only made between two classes of coefficients of friction, i.e.,μ_(N) in the case of wet conditions and μ_(H) in the case of none-wetconditions such as measured by a rain sensor of a wiper actuationcontrol, for example. Further, the 95% sum frequency curve of thefrequency distribution function of the coefficients of friction μ_(G)with 100% slip in the normal case of rating, that is with a singlevehicle with PIARC tires traveling in a longitudinal direction, over allGerman roads, is taken into account to obtain the following times t_(s)illustrated in FIGS. 2a and 2 b. Accordingly, the following time variesin the event of wet conditions from 1.5 seconds to 3.5 seconds and inthe event of none-wet conditions from 0.9 seconds to 1.8 seconds. Thatmeans, the following time is generally within a range which is definedby the minimum limit value t_(s)=0.9 sec. permitted by law and by thetop value t_(s)≈3 seconds limited by filtering-in difficulties, andrises constantly with the increase of driving speed in the range from 30to 150 km/h.

FIG. 3 shows a linking of data, which are normally available in avehicle, intended for the assignment of the instantaneously maximumcoefficient of friction to one of three classes according to a fuzzylogic in conformity with a second embodiment of the present invention.The following data are linked with each other:

1. Parameters of the Tire

The tire profile, the shape of tire, tire compound, type of tire, sizeof tire, etc., as can be taken in particular from the type of tireapplied, is used as tire parameters. The type of tire can either beindicated to an evaluating device manually by the driver, or can be readout automatically. Thus, as is shown in FIG. 4, the type of a tire 2 ona roadway 1 can be determined by way of a magnetic coding 4 on the tire2 by the use of a scanning sensor 5 installed on the spring strut in thearea of brake 3.

2. Parameters of the Roadway

As a parameter of the roadway the road surface temperature is read ininto the evaluating device, for example, by way of a telemetric datainput from transponders planned at the border of roadways, or by way ofsensor means on the vehicle, or simply by way of a constant value.

3. Parameters of the Contact Medium

The parameters of the contact medium between a vehicle and a roadwayhave the greatest influence on the assignment according to thisinvention to a characteristic quantity for the actually existing,instantaneously maximum coefficient of friction μ. It is advisable inthis context to take into account the outside air temperature,preferably measured by an outside temperature sensor of the enginecontrol, the relative humidity, preferably measured by an air humiditysensor of the air conditioning system, the quantity of rain, preferablymeasured by a rain sensor of the wiper actuation control, and the wiperfrequency.

Assigned to each of the parameters is at least one of three values,i.e., H=‘high’, M=‘medium’, or N=‘low’ and, if necessary, 0, as in thecase of a non-existing quantity of rain, or when the wiper is switchedoff, see FIG. 3. An assignment of the coefficient of friction and, thus,of the following time can be effected with respect to one of threecharacteristic quantities based on the above-described parameters, andknowing the wheel tread force in approximation, as calculated from thevehicle weight, center of gravity of the vehicle, axle load distributioncorresponding to the prevailing vehicle deceleration/vehicleacceleration and vehicle speed. According to the embodiment shown inFIG. 3, it is disclosed in the present invention that t_(s) (μ_(H))=1.3sec, t_(s) (μ_(M))=1.8 sec, t_(s) (μ_(N))=2.5 sec.

The logic linking operation according to the present invention can beperformed in an evaluating device, as shown in FIG. 5. Input quantitiesa, b and c can be sent to the evaluating device, and the characteristicquantities for the respective following time t_(s) assigned according tothe present invention are output as output quantities d, for example, toa display or a travel control system. According to the presentinvention, especially the parameters of the tires, of the roadway, andof the contact medium are appropriate as input quantities, as mentionedhereinabove. It should be noted in this respect that in particular thecondition of wear of the tires can be determined by way of a wear modelwhich is obtained by way of a speed histogram in connection with atransverse acceleration histogram and an establishedcoefficient-of-friction histogram, as indicated in FIG. 5 by thecalculation of the various histograms Hi.

Concludingly, it should be mentioned that the transponder system at theborder of roads, which is in the planning stage, will render it possiblein the future to have the parameters of the roadway and the contactmedium directly included as input in an evaluating device. Thisprovision will considerably improve an estimation of the coefficient offriction and, hence, of the following time, and may possibly lead to adifferentiated classification.

The features of the present invention as disclosed in the precedingdescription, in the drawings, and in the claims can be important, bothindividually and in any combination desired, for the realization of thisinvention in its various embodiments.

What is claimed is:
 1. Method of determining a characteristic quantityfor an instantaneously maximum coefficient of friction between the tiresof a vehicle and a roadway, comprising the steps of: i) obtaining dataoutput values from at least one sensor, ii) calculating using said dataoutput values a characteristic quantity of the coefficient of frictionby way of a 95% sum frequency curve of a frequency distribution functionof a plurality of coefficients of friction measured at a 100% slip valueof said vehicle traveling in a longitudinal direction, iii) assigningsaid coefficient of friction to one of a first class in the event of wetroad conditions and a second class in the event of non-wet roadconditions, and iv) presenting the result of step iii), to the vehicledriver or a collision avoidance system of the vehicle, whereby afollowing time of said vehicle in relation to another vehicle that isdirectly in front thereof varies in the first class from 1.5 seconds to3.5 seconds and in the second class from 0.9 seconds to 1.8 seconds. 2.Method of determining a characteristic quantity for an instantaneouslymaximum coefficient of friction between the tires of a vehicle and aroadway, comprising the steps of: i) obtaining data output values fromat least one sensor, ii) calculating using said data output values acharacteristic quantity of the coefficient of friction by way of a 95%sum frequency curve of a frequency distribution function of a pluralityof coefficients of friction measured at a 100% slip value of saidvehicle traveling in a longitudinal direction, iii) assigning saidcoefficient of friction to one of three classes and, thus, to one ofthree characteristic quantities in dependence on one of parameters of atire of the vehicle, parameters of the roadway, and parameters of acontact medium between the tire and the roadway.
 3. Method as claimed inclaim 2, wherein the parameters of the contact medium between the tireand the roadway comprise one of an outside temperature, a relative airhumidity, and a quantity of rain.
 4. Method as claimed in claim 2,wherein the quantity of rain is measured by a rain sensor and actuationof a windshield wiper.
 5. Method as claimed in claim 2, wherein thefirst characteristic quantity represents a low coefficient of friction,the second characteristic quantity represents a medium coefficient offriction, and the third characteristic quantity represents a highcoefficient of friction, and wherein a first following time is assignedto the first characteristic quantity, a second following time isassigned to the second characteristic quantity, and a third followingtime is assigned to the third characteristic quantity, and wherein thefirst, second and third following times are assigned irrespective ofvehicle speed.
 6. Method as claimed in claim 5, wherein the firstfollowing time is 2.5 seconds, the second following time is 1.8 seconds,and the third following time is 1.3 seconds.
 7. Method as claimed in 5,wherein the parameters of the tire of the vehicle comprise one of a typeof tire, a tread design, a tire print, a rubber compound, and acondition of wear of the tire.
 8. Method as claimed in claim 7, whereinthe type of tire is input by the driver or read in automatically by wayof a magnetic coding on the tire.
 9. Method as claimed in claim 7,wherein the condition of wear of the tire is determined by way of a wearmodel, the wear model being determined by way of a speed histogram, atransverse acceleration histogram, and a coefficient of frictionhistogram.
 10. Method as claimed in claim 2, wherein the parameters ofthe contact medium comprise at least one of the height of a water film,ice, and solid snow on the roadway.
 11. Method as claimed in claim 2,wherein at least one of the parameters of the roadway and the parametersof the contact medium are input into an evaluating device by of atransponder system at a border of the roadway.